This invention relates to catheters, and more particularly to balloon catheters.
Certain types of plaques in a patient""s vasculature are likely to rupture. These plaques, once ruptured, are extremely dangerous and can swiftly cause the patient""s death. It is therefore desirable to detect the existence of such high-risk plaques so that they can be disposed of before they rupture.
High-risk plaques are believed to be characterized by large lipid pools hidden behind vascular walls. Because these lipid pools are covered by vascular walls, they cannot be seen by visible light. However, infrared light can penetrate short distances into the vascular wall and can therefore be used to detect such plaques, as well as other intravascular pathology.
A difficulty associated with intravascular use of infrared radiation is that blood absorbs and scatters such radiation. This results in a reduction in the signal-to-noise ratio. As a result, it is desirable to minimize the extent to which infrared radiation propagates through the blood.
One approach to removing blood from a measurement site is to purge or flush the site with saline. This technique provides a short window of opportunity during which a measurement can be taken through the transparent saline. However, once the saline disperses, blood flows back into the measurement site and obscures the vascular wall.
Another approach to removing blood from a measurement site is to displace it with an inflated balloon catheter. However, if the balloon is not sufficiently inflated, considerable blood remains between the balloon and the vascular wall. If the balloon is so inflated that it makes contact with the vascular wall, blood flow is obstructed. This can lead to ischemia at points downstream from the balloon. In addition, the pressure of the balloon on the vascular wall can trigger a rupture of the plaque.
The invention is based on the discovery that if the inflation level, and hence diameter, of a catheter balloon is carefully controlled in real time, the balloon can displace a maximal amount of blood without touching the inner wall of the blood vessel. This reduces scattering and absorption by the blood while avoiding irritation and injury to the inner walls of the blood vessel.
The present invention features catheters for inspecting intravascular structure with infrared radiation. The catheters include balloons that can be inflated to displace blood from the field of view. The extent to which the balloon is inflated is controlled by a feedback loop in which the measured extent of a gap between the outer wall of the balloon and the inner wall of the blood vessel is compared with a desired extent of that gap. The difference between the measured extent and the desired extent provides a basis for either inflating or deflating the balloon.
Another aspect of the invention features a plurality of individually controllable balloons circumferentially disposed around a catheter. A corresponding plurality of measurements provides an estimate of the gap between each of the circumferentially disposed balloons and an arcuate segment of the vascular wall directly opposed from that balloon. By individually controlling each of the balloons, the catheter can be centered within the blood vessel.
In one embodiment, the invention provides an apparatus for controlling an extent of a gap between a wall of a balloon mounted on a catheter and a wall of a lumen into which the catheter is inserted. The apparatus includes a radiation detector or optical fiber mounted within the balloon for generating a feedback signal having information indicative of whether the extent of the gap is greater than or less than a desired value, and a feedback loop for receiving the feedback signal and controlling a size of the balloon to cause the extent of the gap to approach the desired value.
In another embodiment, the invention provides an apparatus having a catheter for insertion into a lumen and a balloon disposed on the catheter. The balloon defines a gap between a wall of the lumen and a wall of the balloon. A radiation source is disposed within the balloon for transmitting radiation through the balloon wall and into coupling fluid present in the gap. The apparatus also includes a feedback loop having a radiation detector or optical fiber disposed within the balloon to receive radiation from the coupling fluid through the balloon wall, and a processor in communication with the radiation detector for determining, on the basis of a signal provided by the radiation detector, a measured extent of the gap. A controller in communication with the processor controls the inflation of the balloon to achieve a desired extent of the gap in response to the measured extent of the gap.
The radiation source can be an infrared emitter and the radiation detector can be an infrared detector. However, the principles of the invention are applicable to emitters and detectors adapted for operation at other frequencies of electromagnetic radiation. In addition, the radiation emitter and detector need not operate at the same frequencies.
The processor can be configured to determine the extent of the gap on the basis of absorption of radiation transmitted by the radiation source, the extent of the absorption being indicative of the extent of the gap. Alternatively, the processor can be configured to determine the extent of the gap on the basis of velocity of coupling fluid in the gap, the velocity of the coupling fluid being indicative of the extent of the gap. In one aspect of the invention, a calibration database in communication with the processor provides information to enable the processor to correct for variations due to wave propagation effects that vary among individuals.
The controller can be configured to control inflation of the balloon by changing a quantity of control fluid in the balloon. The quantity of control fluid can be changed by incremental amounts until the difference between the measured extent of the gap and the desired extent of the gap is within a pre-selected range. Alternatively, the quantity can be changed by an amount that depends on the difference between the measured extent of the gap and the desired extent of the gap.
In another embodiment of the invention, a plurality of balloons is circumferentially disposed around the distal end of the catheter. Each balloon has a size that can be controlled by the controller independently of the other balloons. An embodiment having a plurality of balloons is useful to center the catheter within a lumen or to maintain a spatially constant gap between the wall of each balloon and the wall of the lumen when the cross-section of the lumen is not circular.
The method also includes methods for controlling an extent of a gap between a wall of a balloon mounted on a catheter and a wall of a lumen into which the catheter is inserted.
In one practice, the method includes obtaining a feedback signal having information indicative of whether the extent of the gap is greater than or less than a desired value. In response to the feedback signal, the size of the balloon is controlled to cause the extent of the gap to approach the desired value.
In another practice, the invention includes a method for controlling an extent of a gap between a wall of a balloon catheter and a wall of a lumen, the gap being filled with a coupling fluid. The method includes transmitting first radiation through the coupling fluid and receiving second radiation. The second radiation contains information indicative of propagation effects encountered by the first radiation. On the basis of the second radiation, a measured extent of the gap is determined. The balloon is then inflated to minimize a difference between the measured extent of the gap and a desired extent of the gap.
In one aspect of the invention, the transmitted radiation is infrared radiation. However, the method can include transmitting radiation having any frequency. Similarly, the detected, or received radiation can be infrared radiation. However, the method can include receiving radiation at any frequency. In addition, the frequency of received radiation need not be the same as the frequency of transmitted radiation.
To remove the effect of propagation differences that vary between patients, it is useful to obtain measurements at more than one wavelength. In one aspect, the invention provides for transmitting radiation at a first wavelength at which propagation effects in the coupling fluid are dominated by a first constituent of the coupling fluid. In another aspect, the invention provides for transmitting radiation at a second wavelength at which propagation effects in the coupling fluid are dominated by a second constituent of the coupling fluid. More generally, the invention provides for selecting a plurality of wavelengths and transmitting radiation at each of the plurality of wavelengths.
The received radiation can be attenuated radiation, the extent of the attenuation being indicative of the extent of the gap. Alternatively, the received radiation can be frequency shifted from the transmitted radiation by an amount indicative of the difference between the first and second wavelengths being indicative of velocity of the coupling fluid, the velocity of the coupling fluid being indicative of the extent of the gap.
Because of physiological differences between patients, it is useful to correct for such differences when determining the measured extent of the gap. This can include removing scattering and absorption effects resulting from propagation of the first radiation through the coupling fluid. Alternatively, this can include removing effects from pulsatile flow of the coupling fluid.
In another aspect of the invention, controlling inflation of the balloon catheter includes delivering control fluid to the catheter balloon when the desired extent of the gap exceeds the measured extent of the gap and withdrawing control fluid from the catheter balloon when the measured extent of the gap exceeds the desired extent of the gap. This can be achieved by changing a quantity of control fluid in the catheter balloon incrementally until the difference between the measured extent of the gap and the desired extent of the gap is within a pre-selected range. Alternatively, the quantity of control fluid can be changed by an amount that depends on the difference between the measured extent of the gap and the desired extent of the gap.
In another practice of the invention, transmitting radiation through the coupling fluid includes defining a plurality of radial directions, each of the radial directions extending radially outward from a longitudinal axis of the catheter, and transmitting a corresponding plurality of radiation beams in each of the radial directions. In one aspect, inflation of each of a plurality of balloons circumferentially disposed around the longitudinal axis of the catheter is individually controlled. The extent to which each balloon is inflated is selected to achieve a pre-selected gap between the balloon and the wall of the lumen.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described below. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.
The invention provides many advantages, including a high signal-to-noise ratio resulting from a reduction in the extent to which infrared radiation passes through blood. In addition, the invention reduces the risk of injury to the inner wall of the blood vessel that can result from contact with the balloon.
Other features and advantages of the invention will be apparent from the following detailed description, and from the claims.